Drive Rte 20 from Boston, MA to Seattle, WA.
Take US Route 20 from Boston to Chicago then Rte 66 Chicago to Los Angeles.
Or - I-95 from Boston down through NYC, Philly, Baltimore, DC and then Rte. 50 west across the center of America.
OR... You can let Amtrak do the driving and get a USA Rail Pass. Nothing like a great rail trip in a sleeper with a glass roofed observation car to see the country. almost 5 years ago